The uses of constant current drive circuits are well known. A chief requirement of an "ideal" constant current circuit, by definition, is that the output current produced by the circuit should be a function only of the input current and should be unaffected by any changes in output voltage produced by the load to which the circuit is connected. Such changes in load voltage may occur, for example, when a number of current sources or other devices are coupled to supply current to a common load. In such applications it is often desirable that one source not influence the magnitude of the current provided by another source.
Various techniques are known for producing substantially constant currents notwithstanding load voltage variations. FIG. 1 herein is an example of a known current amplifier 10 (commonly called a "current mirror" amplifier) which provides a moderately constant output current. The circuit includes a "diode-connected" NPN input transistor N1 having a conduction path coupled between an input terminal 1 to which an input current is applied and a common terminal 3 to which a reference or supply voltage V1 is applied. An output NPN transistor N2, having base and emitter electrodes connected to corresponding electrodes of transistor N1, supplies an output current, I out, to an output terminal 2.
In operation, the input current flow though transistor N1 produces a base-emitter voltage which biases the output transistor to provide an output current proportional to the product of the input current and the ratio of the base-emitter junction areas of transistors N1 and N2. For a current gain greater than unity, the junction area of transistor N2 is made greater than that of transistor N1. Conversely, for a current gain less than unity (current attenuation) the junction area of transistor N2 is made less than that of transistor N1. Because of the "early effect" (i.e., base width modulation with collector to base voltage variations), the output current of transistor N2 will vary somewhat with changes in output voltage at terminal 2 produced by the load (not shown) to which the current amplifier circuit 10 is connected. The undesirable sensitivity of the circuit output current to load voltage variations may be reduced to a substantial extent by the known circuit techniques illustrated in FIGS. 2 and 3.
In FIG. 2 an output transistor N4 is connected in cascode with the current mirror amplifier 10 output transistor N2 thereby minimizing the "early effect" by maintaining the collector voltage of transistor N2 at a relatively constant value. This is implemented, as shown, by connecting a diode-connected NPN transistor N3 between a circuit input terminal 4 and the input 1 of current amplifier 10 to generate an offset voltage of 2 Vbe at terminal 4. The offset voltage is applied to the base of NPN transistor N4 having its collector and emitter electrodes connected, respectively, to a circuit output terminal 5 and to the output 2 of current amplifier 10. Thus biased, transistor N4 regulates the collector to emitter voltage of transistor N2 of current amplifier 10 at a relatively constant value of 1-Vbe notwithstanding load voltage variations appearing at the circuit output terminal 5.
In FIG. 3 negative feedback is used for providing suppression of the "early effect" and to improve the output current stability of the current amplifier 10. The example of FIG. 3 differs from that of FIG. 2 in two respects, namely, (i) the connections of terminals 1 and 2 of current amplifier 10 are interchanged and (ii) the junction area ratios of transistors N1 and N2 are reversed (i.e., for a current gain, the junction area of transistor N1 is made larger than that of transistor N2). In operation, the collector emitter voltage of transistor N2 is equal to the sum of the base-emitter voltages of transistors N1 and N4 less that of transistor N3 and therefore is of a relatively constant value notwithstanding load voltage variations at circuit output terminal 5. Feedback regulation results because a tendency for the output current to increase above a value determined by the input current and the base-emitter junction area ratio will cause an increase in voltage across transistor N1. This, in turn, will cause increased conduction through transistor N2 which will reduce the base drive of the output transistor N4 thereby counteracting the assumed increase in the output current.